The present invention, in some embodiments thereof, relates to photodetection and, more particularly, but not exclusively, to detection of long wavelength photons.
Recording and measuring a weak signal presents challenging and acute problems for the designers of modern sensors for myriad applications in diverse fields of science and technology. In these sensors, various primary signals (optical, ultrasonic, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes or ions. Signal charge packets of such elementary charge carriers are amplified and converted to an electrical signal which is fed into a recording or analyzing device and/or used as a feedback signal for monitoring.
One approach to the detection of weak optical signals is the use of photodetectors in which the exposure times are long. These photodetectors typically employ semiconductor technology. Long exposure time photodetectors are suitable for static light source having constant intensity over time (e.g., stars), but are not suitable for rapid imaging applications in which the light has non constant emission intensity and/or originate from moving objects.
Another approach employs avalanche amplification (multiplication) of charge carriers. To date, avalanche amplification is recognized as a highly sensitive and high-speed method of amplification. Avalanche amplification is based on impact ionization arising in a strong electric field. The charge carriers accelerate in the electric field and ionize the atoms of the working medium of the amplifier, resulting in multiplication of the charge carriers. At a high multiplication factor, however, it is difficult to stabilize the avalanche amplification operating point. Additionally, the internal noise level and the response time grow rapidly with the multiplication factor.
Since the energy of photon is inversely proportional to its wavelength the detection of long wavelength single photons, particularly in the infrared (IR) range, is more difficult.
IR detectors can be categorized according to the transport direction, the type of optical transitions, and the type of detection mechanism which can be photovoltaic or photoconductive. Broadly speaking, in response to light impinging on the detector, a photovoltaic detectors generates a measurable voltage (and current), while a photoconductive detector changes its conductance (or resistance).
Currently, prevalent infrared photodetection technology is based on interband (IB) absorption, wherein IB transitions occur in narrow bandgap semiconductors such as HeCdTe, InSb and InGaAs, mostly in PIN configuration. The PIN configuration is typically used in fast detection application. For imaging, the typical configuration is pn junction in case of photo-voltaic devices, and p or n resistor in case of photoconductor. Another technology is based on intersubband (ISB) transitions in heterostructures in a configuration known as Quantum Well Infrared Photodetectors (QWIP), wherein the photodetection mechanism is via absorption between subbands rather than between the valence and conduction bands. An additional technology is based on type-II superlattice structures engineered by deposition of a stack of successive semiconductor layers.
For detection of wavelengths longer from 2 microns, cryogenic temperatures (typically from 77K to 200K) or low temperatures (typically from 200K to 280K) are required for all type of quantum detectors (IB, ISB and superlattice) due to S/N considerations. In general IB detectors can work at higher temperature than ISB detectors.